Method of judging the correctness or incorrectness of a prospective contour point of an irradiation field

ABSTRACT

An image signal representing a radiation image is detected from a recording medium which has been exposed to radiation over a limited irradiation field in order to record the radiation image thereon. From the image signal, a prospective contour point, which is considered to be present on a contour of the irradiation field, is detected. A method for judging the correctness or incorrectness of a prospective contour point of an irradiation field comprises the steps of investigating whether the prospective contour point, which has been detected, satisfies or does not satisfy a predetermined judgment standard, and judging that the detected prospective contour point is incorrect and is not present on the contour of the irradiation field in cases where the detected prospective contour point does not satisfy the predetermined judgment standard.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for judging the correctness orincorrectness of a prospective contour point, which has been detected asbeing present on a contour of an irradiation field on a recordingmedium, in order to recognize where an irradiation field lies on arecording medium in the course of reading out a radiation image whichhas been recorded on the recording medium such as a stimulable phosphorsheet.

2. Description of the Prior Art

Techniques for reading out a recorded radiation image in order to obtainan image signal, carrying out appropriate image processing on the imagesignal, and then reproducing a visible image by use of the processedimage signal have heretofore been known in various fields. For example,as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-rayimage is recorded on an X-ray film having a small gamma value designedfor the type of image processing to be carried out, the X-ray image isread out from the X-ray film and converted into an electric signal, andthe electric signal (image signal) is image-processed and then used whenthe X-ray image is reproduced as a visible image on a copy photograph orthe like. In this manner, a visible image having good image quality andexhibiting such characteristics as high contrast, high sharpness, highgraininess or the like can be reproduced.

Also, when certain kinds of phosphors are exposed to radiation such asX-rays, α-rays, β-rays, γ-rays, cathode rays or ultraviolet rays, theystore part of the energy of the radiation. Then, when the phosphor whichhas been exposed to the radiation is exposed to stimulating rays such asvisible light, light is emitted by the phosphor in proportion to theamount of energy stored during exposure to the radiation. A phosphorexhibiting such properties is referred to as a stimulable phosphor. Asdisclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395,it has been proposed to use stimulable phosphors in radiation imagerecording and reproducing systems. Specifically, a sheet provided with alayer of the stimulable phosphor (hereinafter referred to as astimulable phosphor sheet) is first exposed to radiation which haspassed through an object such as the human body in order to store aradiation image of the object thereon, and is then scanned withstimulating rays, such as a laser beam, which cause it to emit light inproportion to the amount of energy stored during exposure to theradiation. The light emitted by the stimulable phosphor sheet uponstimulation thereof is photoelectrically detected and converted into anelectric image signal, which is used when the radiation image of theobject is reproduced as a visible image on a recording material such asphotographic film, a display device such as a cathode ray tube (CRT), orthe like.

A radiation image recording and reproducing system using a stimulablephosphor sheet is advantageous over conventional radiography usingsilver halide in that the amount of light emitted by the stimulablephosphor sheet is proportional to the energy intensity of the radiation,to which the stimulable phosphor sheet is exposed when an image isrecorded thereon, and the energy intensity of said radiation may beselected from a very wide range (latitude) of radiation energyintensities. If an appropriate read-out gain is selected and used whenthe light emitted by said stimulable phosphor sheet is being detected, adesirable density can be obtained in the finally reproduced visibleimage regardless of the energy intensity of the radiation to which thestimulable phosphor sheet was exposed.

In order to detect an image signal accurately, certain factors whichaffect the image signal must be set in accordance with the dose ofradiation delivered to the stimulable phosphor sheet and the like. Anovel radiation image recording and reproducing system which accuratelydetects an image signal has been proposed in, for example, U.S. Pat. No.4,527,060. The proposed radiation image recording and reproducing systemis constituted such that a preliminary read out operation (hereinaftersimply called "preliminary read out") is carried out for approximatelyascertaining the radiation image stored on the stimulable phosphorsheet. In the preliminary read out the stimulable phosphor sheet isscanned with a light beam having a comparatively low energy level, and apreliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read out operation (hereinaftersimply called "final read out") is carried out for obtaining the imagesignal, which is to be used during the reproduction of a visible image.In the final read out the stimulable phosphor sheet is scanned sheetwith a light beam having an energy level higher than the energy level ofthe light beam used in the preliminary read out, and the radiation imageis read out with the factors affecting the image signal adjusted toappropriate values on the basis of the results of an analysis of thepreliminary read-out image signal.

The term "read-out conditions" as used hereinafter means genericallyvarious factors, which are adjustable and which affect the relationshipbetween the amount of light emitted by the stimulable phosphor sheetduring image read out and the output of a read-out means, i.e. the shapeof the image signal. For example, the term "read-out conditions"includes the value of the read-out gain and scale factor which definethe relationship between the input to the read-out means and the outputtherefrom. The term might also include the power of the stimulating raysused for image read out.

The term "energy level of a light beam" as used herein means the levelof energy of the light beam to which the stimulable phosphor sheet isexposed per unit area. In cases where the energy of the light emitted bythe stimulable phosphor sheet depends on the wavelength of the exposinglight beam, i.e. the sensitivity of the phosphor sheet to the exposinglight beam depends upon the wavelength of the exposing light beam, theterm "energy level of a light beam" means that the energy level of thelight beam, to which the stimulable phosphor sheet is exposed per unitarea, is weighted with the sensitivity of the phosphor sheet to thewavelength of the exposing light beam. In order to change the energylevel of a light beam, light beams of different wavelengths may be used,the intensity of the light beam produced by a laser beam source or thelike may be changed, or the intensity of the light beam may be changedby moving an ND filter or the like into and out of the optical path ofthe light beam. Alternatively, the diameter of the light beam may bechanged in order to alter the scanning density, or the speed with whichthe stimulable phosphor sheet is scanned with the light beam may bechanged.

Regardless of whether the preliminary read out is or is not carried out,it has also been proposed to analyze the image signal (including thepreliminary read-out image signal) obtained and to adjust an imageprocessing condition, which is used when the image signal is processed,on the basis of the results of the analysis of the image signal. Theproposed method is applicable to cases where an image signal is obtainedfrom a radiation image recorded on a recording medium such asconventional X-ray film, as well as to the systems using stimulablephosphor sheets.

Various methods have been proposed for calculating how the read-outconditions for final read out and/or the image processing conditionshould be adjusted on the basis of an analysis of the image signal(including the preliminary read-out image signal). As one of suchmethods, it has been proposed in, for example, U.S. Pat. No. 4,682,028to create a histogram of the image signal. When a histogram of the imagesignal is created, the characteristics of a radiation image recorded ona recording medium such as a stimulable phosphor sheet or an X-ray filmcan be ascertained based on, for example, the maximum value of the imagesignal, the minimum value of the image signal, or the value of the imagesignal at which the histogram is maximum, i.e. the value which occursmost frequently. Therefore, if the read-out conditions for the finalread out, such as the read-out gain or the scale factor, and/or theimage processing condition such as the gradation processing condition orthe frequency response processing condition are based on an analysis ofthe histogram of the image signal, it becomes possible to reproduce avisible image suitable for viewing, particularly for diagnosticpurposes.

On the other hand, in the course of radiation image recording, it isoften desirable for portions of the object not related to a diagnosis orthe like to be prevented from being exposed to radiation. Further, whenthe object portions not related to a diagnosis or the like are exposedto radiation, the radiation is scattered by such portions to the portionthat is related to a diagnosis or the like, and the contrast andresolution are adversely affected by the scattered radiation. Therefore,when a radiation image is recorded on the recording medium, anirradiation field stop is often used for limiting the irradiation fieldto an area smaller than the overall recording region of the recordingmedium so that radiation is irradiated only to that portion of theobject which is to be viewed.

However, in cases where the read-out conditions for the final read outand/or an image processing condition are calculated on the basis of theresults of an analysis of the image signal in the manner described aboveand the image signal is detected from a recording medium, on which aradiation image has been recorded by limiting the irradiation field, theradiation image cannot be ascertained accurately if the image signal isanalyzed without taking the shape and location of the irradiation fieldinto consideration. As a result, incorrect read-out conditions and/or anincorrect image processing condition is set, so that a visible radiationimage suitable for viewing, particularly for diagnostic purposes, cannotbe reproduced.

In order to eliminate the aforesaid problem, it is necessary torecognize the shape and location of an irradiation field and then tocalculate the read-out conditions for the final read out and/or an imageprocessing condition on the basis of the image signal representing imageinformation stored in the region inside of the irradiation field.

The applicant has proposed various methods for recognizing anirradiation field as disclosed in, for example, U.S. patent applicationSer. No. 760,862. The proposed methods allow the aforesaid problem to beeliminated by recognizing where the irradiation field lies on therecording medium, and calculating the readout conditions for the finalread out and/or an image processing condition on the basis of only animage signal corresponding to the region thus recognized.

In general, in the disclosed methods for recognizing an irradiationfield, several points which are considered to be present on a contour ofthe irradiation field, i.e. several prospective contour points, aredetected Thereafter, the straight lines or curves connecting theprospective contour points are detected, and the region surrounded bythe straight lines or curves is recognized as the irradiation field.

A novel method for detecting a prospective contour point has beenproposed in, for example, U.S. patent application Ser. No. 760,862. Theproposed method comprises the steps of reading out a radiation imagewhich has been recorded on a recording medium in order to obtain animage signal, sampling and digitizing the image signal so that a digitalimage signal component represents the image information at each positionof a predetermined number of positions on the recording medium, andcarrying out differentiation processing of the digital image signalcomponents representing image information stored at positions locatedalong a single line on the recording medium. Points at which theabsolute value of the differentiated values obtained during thedifferentiation processing exceed a predetermined threshold value aredetected as prospective contour points. In cases where several suchpoints are present, the point nearest to an edge of the recording mediumis detected as a prospective contour point. Further, another method ofrecognizing the irradiation field has been proposed in copending U.S.patent application Ser. No. 182,685. In the proposed method, theirradiation field is recognized by obtaining digital image data for aplurality of positions on the stimulable phosphor sheet from the imagesignals, detecting prospective edge points, which are considered to beedge portions of the irradiation field on the stimulable phosphor sheet,on the basis of the image data of positions radially outwardly arrangedin a plurality of directions from a predetermined point inside theirradiation field, and recognizing as the irradiation field the regionsurrounded by the lines passing through the prospective edge points.Alternatively, a prospective contour point may be detected by, forexample, a method utilizing pattern matching, or a method wherein astraight line is applied and the contour of an irradiation field isdiscriminated from an inclination of the straight line.

However, in cases where the portion of an image, such as the image ofthe edge of a bone, at which the image density changes sharply as italso does at the contour of an irradiation field, is present in aradiation image, or energy from scattered radiation has been stored inthe region outside of an irradiation field on a recording medium, aprospective contour point is often detected incorrectly. It is difficultto completely eliminate the incorrect detection of a prospective contourpoint. However, if it were possible to determine that a prospectivecontour point had been detected incorrectly, said prospective contourpoint could be canceled, or a correction could be made so that aprospective contour point detected by a different method were employed,instead of said prospective contour point detected incorrectly. In thismanner, an irradiation field could be prevented from being recognizedincorrectly.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a method whichaccurately judges whether a prospective contour point of an irradiationfield, which point has been detected, is or is not correct.

Another object of the present invention is to provide a method forjudging the correctness or incorrectness of a prospective contour pointof an irradiation field, which enables a visible radiation imagesuitable for viewing, particularly for diagnostic purposes to bereproduced.

The present invention provides, in a method for detecting an imagesignal representing a radiation image from a recording medium which hasbeen exposed to radiation over a limited irradiation field in order torecord the radiation image thereon, and detecting a prospective contourpoint, which is considered to be present on a contour of the irradiationfield, from the image signal,

a first method for judging the correctness or incorrectness of aprospective contour point of an irradiation field, which comprises thesteps of:

(i) investigating whether a prospective contour point, which has beendetected, satisfies or does not satisfy a predetermined judgmentstandard, and

(ii) in cases where said detected prospective contour point does notsatisfy the predetermined judgment standard, judging that said detectedprospective contour point is incorrect and is not present on a portionof an irradiation field.

The present invention also provides, in a method for detecting an imagesignal representing a radiation image from a recording medium which hasbeen exposed to radiation by limiting an irradiation field in order torecord the radiation image thereon, and detecting a prospective contourpoint, which is considered to be present at a contour portion of theirradiation field, from the image signal,

a second method for judging the correctness or incorrectness of aprospective contour point of an irradiation field, which comprises thesteps of:

(i) investigating whether a prospective contour point, which has beendetected, satisfies or does not satisfy a predetermined judgmentstandard,

(ii) calculating the mean image density in the region extending fromsaid detected prospective contour point to a point, which is assumed tobe present on a contour of an irradiation field from the positions ofother prospective contour points, and

(iii) in cases where said detected prospective contour point does notsatisfy the predetermined judgment standard and, at the same time, themean image density is lower than a predetermined threshold value,judging that said detected prospective contour point is incorrect and isnot present on the contour of the irradiation field.

The judgment standard may be, for example, the relationship between thedistance from a prospective contour point, which is subjected tojudgment, to a predetermined point inside of an image and distances fromother prospective contour points to said predetermined point.Alternatively, the judgment standard may be the relationship between thedistances from a prospective contour point, which is subjected tojudgment, to other prospective contour points and a predeterminedthreshold value. The judgment standard may also be the relationshipbetween an angle, which is made between two straight lines connecting aprospective contour point, which is subjected to judgment, to two otherprospective contour points and a predetermined threshold value.

The first and second methods for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention investigate whether a prospectivecontour point, which has been detected by a method using differentiationprocessing or the like, is or is not correct. Also, the second methodfor judging the correctness or incorrectness of a prospective contourpoint of an irradiation field in accordance with the present inventioncalculates the mean image density in a region extending from theprospective contour point, which has been detected, to a point which isassumed to be present on a contour of an irradiation field from thepositions of other prospective contour points, and investigates whetherthe prospective contour point, which has been detected, is or is notpresent in a region outside of the irradiation field. Therefore, withthe first and second methods for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention, the correctness or incorrectnessof a prospective contour point can be judged accurately, and it ispossible to prevent an incorrect prospective contour point from beingused to recognize the shape and location of an irradiation field.Accordingly, information about an object can be ascertained accuratelybecause the irradiation field is accurately recognized, and the read-outconditions for the final read out can be adjusted to appropriate values.An appropriate type of image processing can also be carried out. As aresult, it is possible to reproduce a visible radiation image suitablefor viewing, particularly for diagnostic purposes.

The present invention further provides, in a method for detectingprospective contour points which comprises the steps of, in cases where,in order to record a radiation image thereon, a rectangular recordingmedium has been exposed to radiation over a limited irradiation fieldhaving a rectangular shape so that two portions of the contour of theirradiation field are approximately parallel to each other and spaced atapproximately equal distances from two opposite sides of the recordingmedium:

(i) detecting an image signal representing said radiation image fromsaid recording medium,

(ii) sampling and digitizing the image signal to obtain digital imagesignal components representing image information stored at respectivepositions on said recording medium,

(iii) carrying out differentiation processing on the digital imagesignal components representing image information stored at positions ofthe recording medium located along a single line which intersects saidtwo opposite sides of said recording medium at right angles, and

(iv) based on differentiated values obtained from the differentiationprocessing, detecting two prospective contour points each of which isconsidered to be present at each of two said contour portions,

a third method for judging the correctness or incorrectness of aprospective contour point of an irradiation field, which comprises thesteps of:

(a) investigating whether the relationships expressed as

    |l.sub.1 -l.sub.2 |<α, l.sub.3 >β

are or are not satisfied, where l1 denotes the distance from one of saidtwo opposite sides of said recording medium to the detected prospectivecontour point, which is nearer to said one of said two opposite sides ofsaid recording medium than the other of two said detected prospectivecontour points, l2 denotes the distance from the other of said twoopposite sides of said recording medium to the detected prospectivecontour point, which is nearer to said other of said two opposite sidesof said recording medium than the other of two said detected prospectivecontour points, l3 denotes the distance between two said detectedprospective contour points, and α and β each denote a predeterminedthreshold value, and

(b) in cases where at least one of said relationships is not satisfied,judging that two said detected prospective contour points are incorrectand are not present on the contour of the irradiation field.

In cases where a radiation image has been recorded on a rectangularrecording medium over a limited irradiation field having a rectangularshape so that two contour portions of the irradiation field areapproximately parallel to each other and spaced at approximately equaldistances from two opposite sides of the recording medium, twoprospective contour points detected on a single line of the recordingmedium must be spaced approximately equal distances from the sides ofthe recording medium. Therefore, when the relationship expressed as

    |l.sub.1 -l.sub.2 |<α

is not satisfied, i.e. when the distances l1 and l2 are much differentfrom each other, it is recognized that at least one of the two detectedprospective contour points is incorrect.

On the other hand, the image of an edge of a bone or the like in aradiation image is often detected as a prospective contour point. Insuch cases, the relationship expressed as

    |l.sub.1 -l.sub.2 |<α

is often satisfied. However, in such cases, the distance between twodetected prospective contour points is shorter than the distance betweentwo correct prospective contour points. Therefore, when the relationshipexpressed as

    l.sub.3 >β

is not satisfied, it is recognized that the two detected prospectivecontour points are incorrect.

The third method for judging the correctness or incorrectness of aprospective contour point of an irradiation field in accordance with thepresent invention investigates whether a pair of prospective contourpoints detected by a method using differentiation processing are or arenot present at approximately equal distances from two opposite sides ofa rectangular recording medium, and are or are not too close to eachother. Therefore, the third method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention can accurately judge whetherprospective contour points, which have been detected for a rectangularirradiation field limited so that its center line coincides with thecenter line of the rectangular recording medium, are or are not correct.Accordingly, with the third method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention, the same effects as with thefirst and second methods for judging the correctness or incorrectness ofa prospective contour point of an irradiation field in accordance withthe present invention can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a radiation image recording andreproducing system wherein an embodiment of the first method for judgingthe correctness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention is employed,

FIG. 2 is an explanatory view showing the state of a radiation imagestored on a stimulable phosphor sheet in the first method for judgingthe correctness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention,

FIG. 3 is an explanatory view showing how differentiation processing canbe carried out in the first method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention,

FIG. 4A is a graph showing the distribution of image signal componentsin the first method for judging the correctness or incorrectness of aprospective contour point of an irradiation field in accordance with thepresent invention,

FIG. 4B is a graph showing the distribution of the difference valuescalculated from neighboring image signal components in the first methodfor judging the correctness or incorrectness of a prospective contourpoint of an irradiation field in accordance with the present invention,

FIG. 5 is a block diagram showing part of the radiation image recordingand reproducing system shown in FIG. 1,

FIG. 6 is an explanatory graph showing a method for detecting straightlines which connect prospective contour points of an irradiation field,

FIG. 7 is an explanatory view showing a method for extracting a regionsurrounded by straight lines which connect prospective contour points ofan irradiation field,

FIGS. 8, 9, 10, 11 and 12 are explanatory views showing examples of howthe differentiation processing can be carried out in the first methodfor judging the correctness or incorrectness of a prospective contourpoint of an irradiation field in accordance with the present invention,

FIGS. 13, 14, 15, 16 and 17 are explanatory views showing judgmentstandards used in the first method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention,

FIG. 18 is a block diagram showing part of the radiation image recordingand reproducing system shown in FIG. 1, wherein an embodiment of thesecond method for judging the correctness or incorrectness of aprospective contour point of an irradiation field in accordance with thepresent invention is employed,

FIG. 19 is an explanatory view showing the mean image density calculatedand used in the second method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention,

FIG. 20 is an explanatory view showing how the differentiationprocessing is carried out in the third method for judging thecorrectness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention,

FIG. 21 is an explanatory view showing the state of a radiation imagestored on a stimulable phosphor sheet in the third method for judgingthe correctness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention,

FIG. 22A is a graph showing the distribution of image signal componentsin the third method for judging the correctness or incorrectness of aprospective contour point of an irradiation field in accordance with thepresent invention,

FIG. 22B is a graph showing the distribution of the difference valuescalculated from neighboring image signal components in the third methodfor judging the correctness or incorrectness of a prospective contourpoint of an irradiation field in accordance with the present invention,

FIG. 23 is a block diagram showing part of the radiation image recordingand reproducing system shown in FIG. 1, wherein an embodiment of thethird method for judging the correctness or incorrectness of aprospective contour point of an irradiation field in accordance with thepresent invention is employed,

FIG. 24A is a graph showing a different example of the distribution ofimage signal components in the third method for judging the correctnessor incorrectness of a prospective contour point of an irradiation fieldin accordance with the present invention,

FIG. 24B is a graph showing a different example of the distribution ofdifference values calculated from neighboring image signal components inthe third method for judging the correctness or incorrectness of aprospective contour point of an irradiation field in accordance with thepresent invention,

FIG. 25A is a graph showing a further example of the distribution ofimage signal components in the third method for judging the correctnessor incorrectness of a prospective contour point of an irradiation fieldin accordance with the present invention,

FIG. 25B is a graph showing a further example of the distribution ofdifference values calculated from neighboring image signal components inthe third method for judging the correctness or incorrectness of aprospective contour point of an irradiation field in accordance with thepresent invention,

FIG. 26 is an explanatory view showing the third method for judging thecorrectness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention, and

FIG. 27 is an explanatory view showing a method for extracting a regionsurrounded by straight lines which connect prospective contour points ofan irradiation field.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinbelow be described in further detailwith reference to the accompanying drawings.

With reference to FIG. 1, a radiation image recording and reproducingsystem wherein an embodiment of the first method for judging thecorrectness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention is employedcomprises basically a radiation image recording section 20, apreliminary read-out section 30, a final read-out section 40, and animage reproducing section 50. In the radiation image recording section20, radiation 102 is emitted toward an object 101 by a radiation source100 constituted of an X-ray tube or the like. A stimulable phosphorsheet 103 for storing radiation energy thereon is placed at a positionwhere it is exposed to the radiation 102 which has passed through theobject 101, and a radiation image of the object 101 is stored on thestimulable phosphor sheet 103. An irradiation field stop 104 forlimiting the irradiation field of the radiation 102 is disposed betweenthe radiation source 100 and the object 101.

The stimulable phosphor sheet 103 carrying the radiation image of theobject 101 stored thereon is sent to the preliminary read-out section 30by a sheet conveyance means 110 constituted of a conveyor roller or thelike. At the preliminary read-out section 30, a laser beam 202 emanatingfrom a laser beam source 201 first passes through a filter 203 whichfilters out light having wavelengths within the range of wavelengths ofthe light emitted by the stimulable phosphor sheet 103 upon stimulationthereof by the laser beam 202. Then, the laser beam 202 isone-dimensionally deflected by a light deflector 204 such as agalvanometer mirror and directed onto the stimulable phosphor sheet 103by a plane reflection mirror 205. The laser beam source 201 is selectedso that the laser beam 202 emanating therefrom has a wavelengthdistribution different from and far apart from the wavelengthdistribution of the light emitted by the stimulable phosphor sheet 103when it is stimulated. While the laser beam 202 impinges upon thestimulable phosphor sheet 103, the stimulable phosphor sheet 103 ismoved in the direction indicated by the arrow 206 (i.e. in thesub-scanning direction) by a sheet conveyance means 210 constituted ofconveyor rollers or the like and thus the overall surface of thestimulable phosphor sheet 103 is exposed to and scanned by the laserbeam 202. The power of the laser beam source 201, the beam diameter ofthe laser beam 202, the speed with which the laser beam 202 scans, andthe speed at which the stimulable phosphor sheet 103 moves are selectedso that the level of the stimulation energy of the laser beam 202 usedduring the preliminary read-out is lower than the level of thestimulation energy of the laser beam used during the final read-outcarried out in the final read-out section 40.

When it is exposed to the laser beam 202 as described above, thestimulable phosphor sheet 103 emits light in an amount proportional tothe amount of energy stored thereon during exposure to the radiation,and the emitted light enters a light guide member 207 which may be ofthe shape and material disclosed in U.S. Pat. No. 4,346,295. The lightis guided inside of the light guide member 207 through total reflection,emanates from a light output face of the light guide member 207 and isreceived by a photodetector 208 constituted of a photomultiplier or thelike. The light receiving face of the photodetector 208 is positioned sothat it is close contact with a filter which transmits only light havingwavelengths within the range of wavelengths of light emitted by thestimulable phosphor sheet 103 and filters out light having wavelengthswithin the range of wavelengths of the stimulating rays, so that thephotodetector 208 detects only the light emitted by the stimulablephosphor sheet 103 upon stimulation thereof. The light detected by thephotodetector 208 is converted into an electric signal carrying theimage input information stored on the stimulable phosphor sheet 103, andamplified by an amplifier 209. The signal generated by the amplifier 209is digitized by an A/D converter 211, and sent as a preliminary read-outimage signal Sp to a final read-out control circuit 314 in the finalread-out section 40. On the basis of the image input information whichthe preliminary read-out image signal Sp represents, the final read-outcontrol circuit 314 calculates a read-out gain setting value (a), ascale factor setting value (b), and a reproduced image processingcondition setting value (c). The preliminary read-out image signal Sp isalso sent to an irradiation field recognition circuit 220 which will bedescribed in detail later.

After the preliminary read-out from the stimulable phosphor sheet 103 isfinished, the stimulable phosphor sheet 103 is sent to the finalread-out section 40. In this section, a laser beam 302 emanating from alaser beam source 301 first passes through a filter 303 which filtersout light having wavelengths within the range of the wavelengths oflight emitted by the stimulable phosphor sheet 103 upon stimulationthereof by the laser beam 302. Then, the beam diameter of the laser beam302 is precisely adjusted by a beam expander 304. The laser beam 302 isthen deflected by a light deflector 305 formed of a galvanometer mirroror the like, and is made to impinge upon the stimulable phosphor sheet103 by a plane reflection mirror 306. Between the light deflector 305and the plane reflection mirror 306 an fθ lens 307 is disposed forkeeping the beam diameter of the laser beam 302 uniform as it scans thestimulable phosphor sheet 103. While the laser beam 302 impinges uponthe stimulable phosphor sheet 103, the stimulable phosphor sheet 103 ismoved in the direction indicated by the arrow 308 (i.e. in thesub-scanning direction) by a sheet conveyance means 320 constituted ofconveyor rollers or the like and, consequently, the overall area of thestimulable phosphor sheet 103 is exposed to and scanned by the laserbeam 302. Upon exposure to the laser beam 302, the stimulable phosphorsheet 103 emits light in proportion to the amount of energy storedthereon during exposure to radiation, and the light emitted enters alight guide member 309 which is made of the same material and has thesame configuration as the light guide member 207 used for thepreliminary read-out. The light emitted by the stimulable phosphor sheet103 is guided inside of the light guide member 309 through repeatedtotal reflection, emanates from the light output face of the light guidemember 309 and is received by a photodetector 310 constituted of aphotomultiplier or the like. The light receiving face of thephotodetector 310 is positioned in close contact with a filter whichselectively transmits only the light having wavelengths within the rangeof wavelengths of light emitted by the stimulable phosphor sheet 103, sothat the photodetector 310 detects only the light emitted thereby.

The output of the photodetector 310, which photoelectrically detects thelight emission representing the radiation image stored on the stimulablephosphor sheet 103, is amplified to an appropriate level by an amplifier311. The gain of the amplifier 311 is adjusted on the basis of theread-out gain setting value (a) calculated by the control circuit 314.The amplified electric signal is fed into an A/D converter 312 whichconverts the electric signal into a digital signal by use of a scalefactor which is adjusted by the scale factor setting value (b) to suitthe width in the fluctuation of the values of the signal. The digitalsignal thus obtained is fed into a signal processing circuit 313, inwhich it is subjected to signal processing (image processing), thenature of which signal processing is based on the reproduced imageprocessing condition setting value (c). After the digital signal isprocessed, a visible radiation image is obtained which is suitable forviewing, particularly for diagnostic purposes.

The processed digital signal is output as a read-out image signal (afinal read out image signal) So. The final read-out image signal Sogenerated by the signal processing circuit 313 is fed into a lightmodulator 401 in the image reproducing section 50. In the imagereproducing section 50, a laser beam 403 emanating from a reproducinglaser beam source 402 is modulated by the light modulator 401 on thebasis of the final read-out image signal So received from the signalprocessing circuit 313, and is made to impinge upon a photosensitivematerial 405 such as photographic film by a scanning mirror 404 whichcauses the laser beam 403 to scan the photosensitive material 405. Atthis time, the photosensitive material 405 is moved in a directionnormal to the aforesaid scanning direction, i.e. in the directionindicated by the arrow 406. Accordingly, the radiation image representedby the final read-out image signal So is recorded on the photosensitivematerial 405. To reproduce the radiation image, it is possible to useany other appropriate method such as the aforesaid method using a CRTdisplay unit.

A technique for accurately adjusting the read-out gain setting value(a), the scale factor setting value (b) and the image processingcondition setting value (c) when the irradiation field B on thestimulable phosphor sheet 103 is limited as shown in FIG. 2 willhereinbelow be described with reference to FIG. 5. As shown in FIG. 5,the control circuit 314 comprises a signal extracting section 350, ahistogram analysis section 351, a read section 352, and a storagesection 353. The aforesaid preliminary read-out image signal Sp is fedinto the signal extracting section 350 which extracts a preliminaryread-out image signal Sp' only within a specified region as will bedescribed later. The preliminary read-out image signal Sp' is sent fromthe signal extracting section 350 to the histogram analysis section 351.The histogram analysis section 351 creates a histogram of thepreliminary read-out image signal Sp', calculates the maximum value ofthe signal, the minimum value of the signal, the signal value whichoccurs most often, i.e. the signal value corresponding to the maximumvalue of the histogram, or the like, and feeds a signal Sr representingthe calculated value into the read section 352. The storage section 353stores the read-out gain setting value (a), the scale factor settingvalue (b), and the image processing condition setting value (c) suitablefor the aforesaid maximum value, minimum value, signal value whichoccurs most often, or the like. The read section 352 reads the settingvalues (a), (b) and (c) suitable for the signal Sr from the storagesection 353, and feeds them respectively into the amplifier 311, the A/Dconverter 312, and the signal processing circuit 313.

How the signal extracting section 350 extracts a signal will now bedescribed below. The irradiation field recognizing circuit 220 iscomposed of a differentiation processing section 221, a threshold valueadjusting section 222, a prospective contour point signal detectingsection 223, a correctness or incorrectness judging section 224, and anoperating section 225. The preliminary read-out image signal Sp is fedinto the differentiation processing section 221 and the prospectivecontour point signal detecting section 223. The differentiationprocessing section 221 differentiates the components of the preliminaryread-out image signal Sp corresponding to positions on the stimulablephosphor sheet 103 located along a line in the direction of D1, thenalong lines in the directions D2, D3, . . . , Dn shown in FIG. 3.Differentiation processing may be of the one-dimensional type of firstor higher order, or may be of the two-dimensional type of first orhigher order. In cases of a discretely sampled image, differentiation isequivalent to calculation of the difference between the values ofneighboring image signal components. In this embodiment, the differencein the values of neighboring image signal components is calculated.Lines along the directions D1 through Dn radiate from the center O ofthe stimulable phosphor sheet 103 toward the edges thereof. In thisembodiment, lines along the directions D1 through Dn radiate at equalangle intervals. For example, if the size of the stimulable phosphorsheet 103 is 256 mm×192 mm, approximately 64 directions are selected asthe directions D1 through Dn. Differentiation processing is carried out,and the differences among image signal components of the preliminaryread-out image signal Sp corresponding to adjacent positions on thestimulable phosphor sheet are calculated. A signal Sm representing thedifferences is stored in the memory 222, and fed into the thresholdvalue adjusting section 223. Based on the signal Sm representing thedifferences and a signal Sth which is received from the threshold valueadjusting section 222 and which represents a threshold value Th, theprospective contour point signal detecting section 223 detects aprospective contour point which is considered to be present on a contourof the irradiation field B on the stimulable phosphor sheet 103.Specifically, the levels of the image signal components of thepreliminary read-out image signal Sp for the region inside of theirradiation field B are distinctly higher than those for the regionoutside of the irradiation field B. Therefore, the values of the imagesignal components of the preliminary read-out image signal Spcorresponding to positions on the stimulable phosphor sheet locatedalong a line in a certain direction Di are distributed as shown in FIG.4A. Accordingly, as shown in FIG. 4B, the values of the aforesaiddifferences change markedly at an edge of an irradiation field. Theprospective contour point signal detecting section 223 detects a point,at which the difference has a negative sign and the absolute value ofwhich difference exceeds the threshold value Th, as a prospectivecontour point.

Thereafter, the prospective contour point signal detecting section 223extracts image signal components at prospective contour points, whichhave been detected in the manner described above, from the preliminaryread-out image signal Sp. The prospective contour point signal detectingsection 223 finds the positions of picture elements, respectivelycorresponding to the extracted image signal components, and feeds asignal Se representing the positions of the picture elements into thecorrectness or incorrectness judging section 224. Most of the imagesignal components extracted from the preliminary read-out image signalSp constitute an image signal, which represents the contour of theirradiation field B on the stimulable phosphor sheet 103 as shown inFIG. 2. In this embodiment, as shown in FIG. 2, the positions of thepicture elements are expressed on an x-y orthogonal coordinate system onthe stimulable phosphor sheet 103.

Hereinafter, the term "directions of differentiation processing" meansthe directions of the lines along which positions on the stimulablephosphor sheet lie, the image signal components representing the imageinformation at said positions undergoing differentiation processingstarting with the image signal components representing image informationat positions at one end of the line.

The correctness or incorrectness judging section 224 judges whether theprospective contour points at the positions of the picture elementswhich are represented by the signal Se are or are not truly present on acontour of the irradiation field. Specifically, with reference to FIG.13, Em denotes a prospective contour point which is to be subjected tojudgment, and Em+1 and Em-1 denote prospective contour points which arepresent beside the prospective contour point which is to be subjected tojudgment. The correctness or incorrectness judging section 224investigates whether the distance l from the center point O of thestimulable phosphor sheet 103 to the prospective contour point Em, thedistance l1 from the center point O to the prospective contour pointEm+1, and the distance l2 from the center point O to the prospectivecontour point Em-1 satisfy or do not satisfy the relationship expressedas ##EQU1## where n denotes the number of directions of differentiationprocessing. As in ordinary circumstances, the irradiation field B inthis embodiment has a polygonal shape free of any concave-like regionssuch as those in a star-shaped polygon. Also, the irradiation field B islimited so that the center point O of the stimulable phosphor sheet 103is positioned within the irradiation field B. Formula (1) is notsatisfied when, as shown in FIG. 13, the prospective contour point Em ispositioned closer to the center point O of the stimulable phosphor sheet103 than the prospective contour points Em+1 and Em-1. However, insofaras the irradiation field B has a polygonal shape free of concave-likeregions, the prospective contour point Em positioned closer to thecenter point O than the prospective contour points Em+1 and Em-1 cannotbe present on the contour of the irradiation field B. Therefore, whenthe prospective contour point Em does not satisfy Formula (1), thecorrectness or incorrectness judging section 224 judges that theprospective contour point Em is present inside of the irradiation fieldB, and therefore is incorrect. The correctness or incorrectness judgingsection 224 eliminates the information about the picture elementposition of the prospective contour point Em from the picture elementposition signal Se. On the other hand, when the prospective contourpoint Em satisfies Formula (1), the information about the pictureelement position of the prospective contour point Em is not eliminatedfrom the picture element position signal Se. The correctness orincorrectness judging section 224 carries out the aforesaid judgment andeliminating processes, when necessary, for all of the detectedprospective contour points, and feeds a processed picture elementposition signal Se' to the operating section 225.

In this embodiment, when the prospective contour point Em is judged tobe incorrect, the information about the picture element position of thatpoint is canceled from the signal Se. Alternatively, a new prospectivecontour point may be derived from other prospective contour points. Byway of example, another point on the stimulable phosphor sheet 103 whichlies on a line passing through the center point O and the incorrectcontour point Em and which is spaced at a distance l' expressed as##EQU2## from the center point O of the stimulable phosphor sheet 103may be employed as the aforesaid new prospective contour point.

After the prospective contour points are detected in the mannerdescribed above, lines connecting them may be recognized as the contourof the irradiation field B. The lines connecting the prospective contourpoints can be found by using one of several methods, for example, amethod wherein prospective contour points remaining after a smoothingprocess has been carried out are connected together, a method wherein aplurality of straight lines are found by locally applying the method ofleast squares and the straight lines are then connected together, or amethod wherein a spline curve or the like is applied. In thisembodiment, the operating section 225 finds a plurality of straightlines connecting the prospective contour points by utilizing a Houghtransformation. The processing done to find the straight lines willhereinbelow be described in detail.

If (xo,yo) are the coordinates of the picture element positions (theprospective contour points) which the signal Se' represents, then theoperating section 225 calculates the curves expressed as

    ρ=xo cos θ+yo sin θ

xo and yo being constant for each prospective contour point coordinate(xo,yo). FIG. 6 shows the curves thus obtained, and the number of curvesequals the number of prospective contour point coordinates (xo,yo).

Then, the operating section 225 calculates the coordinates (ρo,θo) ofthe points where the curves intersect and where the number of curvesintersecting at each point (ρo,θo) is not smaller than a predeterminednumber Q. Because of errors in finding the prospective contour pointcoordinates (xo,yo), many curves rarely intersect exactly at a singlepoint. Therefore, by way of example, in the case where multiple sets oftwo curves have intersections spaced from one another by only smalldistances not longer than a predetermined distance, the point ofintersection at the middle of the group of the intersections is taken asthe aforesaid intersection (ρo,θo). Then, from each intersection(ρo,θo), the operating section 225 calculates a straight line expressedas

    ρo=x cos θo+y sin θo

on the x-y orthogonal coordinate system. The straight line thuscalculated extends along a plurality of the prospective contour pointcoordinates (xo,yo). It often occurs that bone edges or other imageportions at which the image density changes sharply in the irradiationfield B are also detected as prospective contour points. Therefore, asshown in FIG. 2, there is the risk that a straight line such as L willconnect points where the image density changes sharply but which are notcontour points with points actually on the contour of the irradiationfield. However, if the aforesaid predetermined number Q is madesubstantially large (for example, 20 or larger), the straight line L isnot obtained. Instead only straight lines representing the contour ofthe irradiation field are obtained.

In cases where the prospective contour points are distributed as shownin FIG. 2, straight lines as shown in FIG. 7 are obtained. The operatingsection 225 then detects the region surrounded by a plurality ofstraight lines L1, L2, L3, . . . , Ln obtained in this manner, andrecognizes said region as the irradiation field B. Specifically, forexample, the region is recognized in the manner described below. Theoperating section 225 stores line segments M1, M2, M3, . . . , Mmconnecting the corners of the stimulable phosphor sheet 103 with thecenter point G (four line segments in cases where the stimulablephosphor sheet 103 is rectangular), and detects whether or not each ofthe line segments M1 to Mm intersects with each of the straight lines L1to Ln. In cases where an intersection is present, the operating section225 divides the stimulable phosphor sheet 103 into two regions: oneincluding the corner of the stimulable phosphor sheet 103 to which theline segment is connected and delineated by the straight line and theother including the remainder of the stimulable phosphor sheet. Theoperating section 225 then discards the region including the corner.This operation is carried out for all of the straight lines L1 to Ln andthe line segments M1 to Mm, and the region surrounded by the straightlines L1 to Ln is not discarded. The region thus obtained is recognizedas the irradiation field B.

The operating section 225 sends a signal St representing the shape andlocation of the irradiation field B recognized in the manner describedabove to the signal extracting section 350 in the final read-out controlcircuit 314. The signal extracting section 350 extracts the image signalcomponents corresponding to the region, which the signal St represents,from the preliminary read-out image signal Sp, and sends the preliminaryread-out image signal Sp' comprising the extracted image signalcomponents to the histogram analysis section 351. Therefore, thehistogram analysis section 351 carries out an analysis of the histogramof only those image signal components representing the region of thestimulable phosphor sheet 103 that was actually exposed to radiation,and the aforesaid setting values (a), (b) and (c) are made suitable forthe actual image input information.

In the aforesaid embodiment, differentiation processing is started onimage signal components representing image information stored atpositions neighboring the center point O of the stimulable phosphorsheet 103 in the region inside of the irradiation field B. However,differentiation processing may be started on image signal componentsrepresenting image information stored at any position as long as thestarting point lies on the stimulable phosphor sheet 103. For example,in cases where the irradiation field is limited to a very small area,the center point of the stimulable phosphor sheet may be present in theregion outside of the irradiation field. In such cases, differentiationprocessing may be started on image signal components representing imageinformation present at positions of the stimulable phosphor sheet 103which lie in the region inside of the irradiation field, for example, aposition at which the density level is the highest among density levelson the stimulable phosphor sheet, a position at which the center ofgravity of the density is located, or a position at which the center ofgravity in the region on a high density side is located when the imagedensity levels are converted into the two-valued system.

Also, directions D1 through Dn of differentiation processing need notnecessarily be selected at equal angle intervals. For example, as shownin FIG. 8, a plurality of points which lie at equal distance intervalsalong edge portions of the stimulable phosphor sheet 103 may beselected, and directions D1 through Dn from a point P inside of theirradiation field B toward the plurality of said points may be selectedas the directions of differentiation processing.

Furthermore, as shown in FIG. 9, the directions D of differentiationprocessing may have larger intervals between them in a region where thedistance g between a point P inside of the irradiation field B and aprospective contour point E does not change very much, i.e. in theregion corresponding to a region h1. In a region where the distance gchanges markedly, i.e. in the region corresponding to a range h2, thedirections D of differentiation processing may have smaller intervalsbetween them.

A judgment standard different from Formula (1) may be utilized to judgewhether a prospective contour point is or is not correct on the basis ofthe relationship between the distance from the prospective contourpoint, which is to be subjected to judgment, to a predetermined point ina region inside of an image and the distances from different prospectivecontour points to the predetermined point. For example, in cases whereit is known in advance that the shape of the irradiation field B isrectangular as shown in FIG. 14, the correctness or incorrectnessjudging section 224 may investigate whether a prospective contour pointEm different from prospective contour points at extreme portions of eachside of the irradiation field B satisfies or does not satisfy therelationship expressed as ##EQU3## When the prospective contour point Emdoes not satisfy Formula (2), it can be judged to be incorrect. This isbecause, if the prospective contour point Em is correct, it is notlocated closer to an edge of the stimulable phosphor sheet 103 than astraight line connecting prospective contour points Em+1 and Em-1, whichare present on both sides of the prospective contour point Em.

Also, in cases where the shape of the irradiation field B is circular asshown in FIG. 15, the correctness or incorrectness judging section 224may investigate whether a prospective contour point Em, which is to besubjected to judgment, satisfies or does not satisfy the relationshipexpressed as

    |l-l|<α

where l denotes the distance from the prospective contour point Em tothe center point O of the irradiation field B, l denotes the mean valueof the distances from all of the other prospective contour points to thecenter point O of the irradiation field B, and α denotes a predeterminedthreshold value. When the prospective contour point Em does not satisfythe relationship, i.e. when the distance l is much different than thedistances from the other prospective contour points to the center pointO of the irradiation field B, the prospective contour point Em can bejudged to be incorrect.

Alternatively, in cases where the irradiation field B has the shapeshown in FIG. 15, the correctness or incorrectness judging section 224may investigate whether the prospective contour point Em, which is to besubjected to judgment, satisfies or does not satisfy the relationshipexpressed as

    |l-(l.sub.1 +l.sub.2)/2|<β

where l denotes the distance from the prospective contour point Em tothe center point O of the irradiation field B, denotes the distance froma prospective contour point Em+1 to the center point O of theirradiation field B, l2 denotes the distance from a prospective contourpoint Em-1 to the center point O of the irradiation field B, saidprospective contour points Em+1 and Em-1 being present on both sides ofthe prospective contour point Em, and β denotes a predeterminedthreshold value. When the prospective contour point Em does not satisfythe relationship, i.e. when the distance l is much different than themean value of the distances l1 and l2, the prospective contour point Emcan be judged to be incorrect.

Furthermore, in the first method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention, distances between detectedprospective contour points may be utilized as the judgment standard. Forexample, in cases where the shape of the irradiation field B is circularas shown in FIG. 16, the correctness or incorrectness judging section224 may investigate whether a prospective contour point Em, which is tobe subjected to judgment, satisfies or does not satisfy the relationshipexpressed as

    (l.sub.3 +l.sub.4)/2<γ

where l3 denotes the distance from the prospective contour point Em to aprospective contour point Em+1, l4 denotes the distance from theprospective contour point Em to a prospective contour point Em-1, saidprospective contour points Em+1 and Em-1 being present on both sides ofthe prospective contour point Em, and γ denotes a predeterminedthreshold value. When the prospective contour point Em does not satisfythe relationship, i.e. when the mean value of the distances l3 and l4 isvery large, the prospective contour point Em can be judged to beincorrect.

Moreover, in the first method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention, the angle between two straightlines, each line connecting a prospective contour point, which is to besubjected to judgment, to one of two different prospective contourpoints, may be utilized as the judgment standard. For example, in caseswhere the shape of the irradiation field B is circular as shown in FIG.17, straight lines connecting a prospective contour point Em, which isto be subjected to judgment, with prospective contour points Em+1 andEm-1, which are present on both sides of the prospective contour pointEm, are denoted by J1 and J2. The correctness or incorrectness judgingsection 224 investigates whether the prospective contour point Emsatisfies or does not satisfy the relationship expressed as

    θ>Φ

where θ denotes the angle between the straight lines J1 and J2, and Φdenotes a predetermined threshold value. When the prospective contourpoint Em does not satisfy the relationship, i.e. when the angle θ isvery small, the prospective contour point Em can be judged to beincorrect. Alternatively, the correctness or incorrectness judgingsection 224 may calculate such angles for all of the detectedprospective contour points, and may investigate whether the prospectivecontour point Em satisfies or does not satisfy the relationshipexpressed as

    |l- l |<δ

where l denotes the mean value of the angles, and δ denotes apredetermined threshold value. When the prospective contour point Emdoes not satisfy the relationship, i.e. when the angle θ is muchdifferent than the mean value, the prospective contour point Em can bejudged to be incorrect.

An embodiment of the second method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention, which improves the accuracy ofjudgment, will hereinbelow be described with reference to FIG. 18. FIG.18 shows an irradiation field recognizing circuit 250 which is used inthe same manner as the irradiation field recognizing circuit 220 shownin FIG. 5. In FIG. 18, similar elements are numbered with the samereference numerals with respect to FIG. 5. In the irradiation fieldrecognizing circuit 250, the signal Se which represents the pictureelement positions of prospective contour points and which is generatedby the prospective contour point signal detecting section 223 is fedinto a correctness or incorrectness judging section 251. The correctnessor incorrectness judging section 251 also receives the preliminaryread-out image signal Sp. For this embodiment, the shape of theirradiation field B is rectangular as shown in FIG. 14. The correctnessor incorrectness judging section 251 utilizes Formula (2) as thejudgment standard, and investigates whether a prospective contour pointEm satisfies or does not satisfy Formula (2). When the prospectivecontour point Em does not satisfy Formula (2), the correctness orincorrectness judging section 251 judges that the prospective contourpoint Em is located closer to the edge of the stimulable phosphor sheet103 than a line connecting prospective contour points Em+1 and Em-1,which are present on both sides of the prospective contour point Em.With reference to FIG. 19, in cases where the prospective contour pointEm has been judged to be located closer to the edge of the stimulablephosphor sheet 103 than the straight line connecting the prospectivecontour points Em+1 and Em-1, the correctness or incorrectness judgingsection 251 finds a point Em', which is assumed to be present on thecontour of the irradiation field B, on the basis of the prospectivecontour points Em+1 and Em-1. In this embodiment, a point which ispresent on a straight line connecting the prospective contour point Emwith the center point O of the stimulable phosphor sheet 103 and whichis spaced at a distance expressed as ##EQU4## from the center point O isemployed as the point Em'.

Thereafter, from the preliminary read-out image signal Sp, thecorrectness or incorrectness judging section 251 calculates the meanimage density D in the region between the point Em' and the originallydetected prospective contour point Em. If the mean image density D islower than a predetermined threshold value, the correctness orincorrectness judging section 251 judges that the prospective contourpoint Em is incorrect. Specifically, if the prospective contour point Emis present closer to the edge of the stimulable phosphor sheet 103 thanthe straight line connecting the prospective contour points Em+1 andEm-1, the region between the prospective contour point Em and the pointEm' lies outside of the irradiation field B. The image density isgenerally low in such a region. Therefore, judgment can be made asdescribed above. In cases where the prospective contour point Em isjudged to be present in the region outside of the irradiation field B,the correctness or incorrectness judging section 251 cancels theprospective contour point Em, or employs the point Em' as a correctprospective contour point, instead of the prospective contour point Em.

The signal Se' representing the picture element positions of theprospective contour points, which have been judged to be correct, is fedinto the operating section 225. Based on the signal Se', the contour ofthe irradiation field B is detected in the same manner as describedabove.

The step of judging, based on the mean image density, whether a detectedprospective contour point is or is not present in the region outside ofthe irradiation field B may be carried out in cases where correctness orincorrectness of a prospective contour point is judged with a differentjudgment standard as well as in cases where judgment is made by usingFormula (2) as the judgment standard.

In cases where an irradiation field B having the shape shown in FIG. 2is to be recognized, a single prospective contour point on the contourof the irradiation field is generally detected when differentiationprocessing is carried out along a single line. On the other hand, incases where an irradiation field B having a shape shown in FIG. 10 orFIG. 11 is to be recognized, a plurality of prospective contour pointson the contour of the irradiation field may be detected for a singledirection of differentiation processing. In such cases, when all of thepoints at which the values of the aforesaid differences exceed thethreshold value are detected as prospective contour points, theprospective contour points on the contour of the irradiation field canbe detected completely, and an irradiation field B having a complicatedshape can be recognized accurately. Also, as shown in FIG. 12, in caseswhere the shape of the irradiation field B is rectangular,differentiation processing may be carried out by shifting the positionof the point P at which differentiation processing is started. In suchcases, the signs (positive or negative) of the difference values areopposite for points on the contour of the irradiation field on theleft-hand and right-hand sides of the stimulable phosphor sheet 103. Inany case, the absolute value of the difference and a threshold value maybe compared with each other. The first and second methods for judgingthe correctness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention areapplicable also to cases where prospective contour points are detectedin this manner.

An embodiment of the third method for judging the correctness orincorrectness of a prospective contour point of an irradiation field inaccordance with the present invention will be described hereinbelow withreference to FIG. 23. FIG. 23 shows an irradiation field recognizingcircuit 220' which is used in the same manner as the irradiation fieldrecognizing circuit 220 shown in FIG. 5. In FIG. 23, similar elementsare numbered with the same reference numerals with respect to FIG. 5.

With reference to FIG. 23, an irradiation field recognizing circuit 200'is composed of a differentiation processing section 221', a thresholdvalue adjusting section 222', a prospective contour point signaldetecting section 223', a correctness or incorrectness judging section224', and an operating section 225'. The preliminary read out imagesignal Sp is fed into the differentiation processing section 221' andthe prospective contour point signal detecting section 223'. Thedifferentiation processing section 221' differentiates the preliminaryread-out image signal Sp, which has been digitized, along a single lineD1, then along lines D2, D3, . . . , Dn shown in FIG. 20. In thisembodiment, it is known in advance that the irradiation field B has arectangular shape on a stimulable phosphor sheet 103 as shown in FIG.20. The lines D1, D2, D3, . . . , Dn intersect the portions B1 and B2 ofthe contour of the irradiation field B at right angles. The contourportions B1 and B2 are spaced at equal distances from a left edge 103Land a right edge 103R of the stimulable phosphor sheet 103,respectively. Differentiation processing may be of the one-dimensionaltype of first or higher order, or may be of the two-dimensional type offirst or higher order. In cases where the image is discretely sampled,differentiation is equivalent to the calculation of the differencebetween neighboring image signal components. In this embodiment, thedifference between neighboring image signal components is calculated.Differentiation processing is described in detail in U.S. patentapplication Ser. No. 760,862.

A signal Sm representing the differences obtained by differentiationprocessing is fed into the prospective contour point signal detectingsection 223'. Based on the signal Sm representing the differences and asignal Sth which is received from the threshold value adjusting section222' and which represents a threshold value Th, the prospective contourpoint signal detecting section 223' detects prospective contour pointswhich are considered to be present on the contour portions B1 and B2 ofthe irradiation field B on the stimulable phosphor sheet 103.Specifically, the levels of the image signal components of thepreliminary read-out image signal Sp for the region inside of theirradiation field B are distinctly higher than those for the regionoutside of the irradiation field B. Therefore, the values of the imagesignal components of the preliminary read-out image signal Sp along acertain direction Di are distributed as shown in FIG. 22A. Accordingly,as shown in FIG. 22B, the values of the aforesaid differences changemarkedly at the edge of an irradiation field. The prospective contourpoint signal detecting section 223' detects two points, at each of whichthe absolute value of the difference exceeds the threshold value Th, asprospective contour points. In cases where energy from scatteredradiation which was stored in a region outside of the irradiation fieldB is detected during a preliminary read out, the image signal componentsof the preliminary read-out image signal Sp along the line Di are oftendistributed as shown in, for example, FIG. 24A and, as a result, thedifferences between neighboring image signal components are distributedas shown in FIG. 24B. Also, in cases where a radiation image including abone image portion has been recorded on a stimulable phosphor sheet 103,the image signal components of the preliminary read-out image signal Spalong a line Di extending across the image of the edge of the bone areoften distributed as shown in, for example, FIG. 25A and, as a result,the differences between neighboring image signal components aredistributed as shown in FIG. 25B. In such cases, three or more points,at which the absolute values of the differences exceed the thresholdvalue Th, are often detected. Therefore, in such cases, two prospectivecontour points are detected for a single line Di by, for example,selecting two points at which the absolute values of the differences arethe largest and next largest.

Thereafter, the prospective contour point signal detecting section 223'extracts image signal components at the two prospective contour points,which have been detected in the manner described above, from thepreliminary read-out image signal Sp. The prospective contour pointsignal detecting section 223' finds the positions of the pictureelements respectively corresponding to the extracted image signalcomponents, and feeds a signal Se representing the positions of thepicture elements into the correctness or incorrectness judging section224'. Most of the image signal components extracted from the preliminaryread-out image signal Sp constitute an image signal, which representsthe contour portions B1 and B2 of the irradiation field B on thestimulable phosphor sheet 103 as shown in FIG. 20. In this embodiment,as shown in FIG. 20, the positions of the picture elements are expressedon an x-y orthogonal coordinate system on the stimulable phosphor sheet103.

The correctness or incorrectness judging section 224' judges whether theprospective contour points at the positions of the picture elementswhich are represented by the signal Se are or are not truly present atcontour portions B1 and B2 of the irradiation field. Specifically, withreference to FIG. 26, E1 and E2 denote the two prospective contourpoints detected on each line. The correctness or incorrectness judgingsection 224' calculates the distance l1 from the left edge 103L of thestimulable phosphor sheet 103 to the left prospective contour point E1,the distance l2 from the right edge 103R of the stimulable phosphorsheet 103 to the right prospective contour point E2, and the distance l3between the prospective contour points E1 and E2. Thereafter, thecorrectness or incorrectness judging section 224' investigates whetherthe relationships expressed as

    |l.sub.1 -l.sub.2 |<α              (3)

    l.sub.3 >β                                            (4)

where α and β each denote a predetermined threshold value, are or arenot satisfied. The difference between the distances l1 and l2 should besmall. However, if points I and II shown in FIG. 24B were detected asthe prospective contour points E1 and E2, the difference between thedistances l1 and l2 is large, and Formula (3) is not satisfied. Also,the prospective contour points E1 and E2 must be spaced a substantialdistance from each other. However, if points I and II shown in FIG. 25Bwere detected as the prospective contour points E1 and E2, because thepoints I and II are somewhat close to each other, Formula (4) is notsatisfied. Accordingly, when at least one of Formulas (3) and (4) is notsatisfied, the correctness or incorrectness judging section 224' judgesthat the prospective contour points E1 and E2 are incorrect, andeliminates the information about the prospective contour points E1 andE2 from the picture element position signal Se. On the other hand, whenboth Formulas (3) an (4) are satisfied, the information about theprospective contour points E1 and E2 is not eliminated from the pictureelement position signal Se. The correctness or incorrectness judgingsection 224' carries out the aforesaid judgment and eliminatingprocesses, when necessary, for all of the detected prospective contourpoints, and feeds a processed picture element position signal Se' to theoperating section 225'.

In this embodiment, when the prospective contour points E1 and E2 arejudged to be incorrect, they are canceled. Alternatively, newprospective contour points may be derived from other prospective contourpoints.

Based on the signal Se' representing the picture element positions ofcorrect prospective contour points, the operating section 225' detectstwo straight lines connecting the correct prospective contour points.The two straight lines may be recognized as portions of the contour ofthe irradiation field B. The lines connecting the prospective contourpoints can be found by using one of several methods, for example, amethod wherein prospective contour points remaining after a smoothingprocess has been carried out are connected together. It is also possibleto employ a method wherein, as shown in FIG. 27, the number of theprospective contour points E1, E1, . . . and the number of theprospective contour points E2, E2, . . . are counted along the verticallines (i.e. the total number of said points along each of two linesparallel to the vertical axis are counted). The numbers of said pointsoccuring at respective positions of the horizontal axis are then countedand graphed as shown in FIG. 27, and two straight lines which passthrough positions x1 and x2 are detected. The operating section 225'sends the signal St representing the size and location of theirradiation field B recognized in the manner described above to thesignal extracting section 350 of the final read-out control circuit 314.The signal extracting section 350 extracts the image signal componentscorresponding to the image information stored in the region, which thesignal St represents, from the preliminary read out image signal Sp, andsends the preliminary read-out image signal Sp' comprising the extractedimage signal components to the histogram analysis section 351.Therefore, an analysis of a histogram of the image signal is carried outin the histogram analysis section 351 only for the region on thestimulable phosphor sheet 103 that was actually exposed to radiation,and the aforesaid setting values (a), (b) and (c) are adjusted to besuitable for the actual image input information.

In the embodiment shown in FIG. 23, differentiation processing iscarried out only along lines parallel to the x axis as shown in FIG. 20.In cases where, as shown in FIG. 21, the irradiation field B has arectangular shape, prospective contour points may also be detected bycarrying out differentiation processing along several lines parallel tothe y axis. Two straight lines which connect the detected prospectivecontour points and which are parallel to the x axis may then bedetected, and a rectangular region surrounded by the detected straightlines parallel to the x axis and the straight lines which have beendetected in the manner mentioned above and which are parallel to the yaxis may be recognized as enclosing the irradiation field B.

In general, the preliminary read out described above is carried out forpicture elements which are larger than the picture elements of the finalread out. In the aforesaid embodiments of the first, second and thirdmethods for judging the correctness or incorrectness of a prospectivecontour point of an irradiation field in accordance with the presentinvention, differentiation processing may be carried out on image signalcomponents obtained by such a comparatively rough read-out operation.Alternatively, the image signal components may be interpolated to obtainfiner image signal components corresponding to small picture elementsgiving the image a finer resolution, and differentiation processing maybe carried out on this grater number of image signal components. Also,differentiation processing may be conducted for image signal componentsobtained by averaging the values of the image signal components detectedat a plurality of picture elements.

In the radiation image recording and reproducing system shown in FIG. 1,the preliminary read-out section and the final read-out section aredisposed independently. However, as disclosed in, for example, U.S. Pat.No. 4,527,060, a single read-out system may be used for the preliminaryread out and the final read out. In this case, after the preliminaryread out is finished, the stimulable phosphor sheet is returned to theread-out system by a sheet conveyance means and then the final read outis carried out. In the preliminary read-out step, the energy level ofthe stimulating rays is adjusted by a stimulating ray energy adjustingmeans so that it is lower than the energy level of the stimulating raysused in the final read out. The first, second and third methods forjudging the correctness or incorrectness of a prospective contour pointof an irradiation field in accordance with the present invention arealso applicable to such cases.

Also, in the first, second and third methods for judging the correctnessor incorrectness of a prospective contour point of an irradiation fieldin accordance with the present invention, instead of recognizing theshape and location of an irradiation field based on the preliminaryread-out image signal, a prospective contour point of an irradiationfield may be recognized by utilizing the final read-out image signal oran image signal, which is obtained by directly carrying out an imageread-out step corresponding to final read out. In this case, informationon the recognized irradiation field can be utilized for, for example,adjusting the image processing condition setting value (c).

Furthermore, the first, second and third methods for judging thecorrectness or incorrectness of a prospective contour point of anirradiation field in accordance with the present invention are alsoapplicable to cases where a radiation image is read out from a recordingmedium such as silver halide photographic film on which an X-ray imagehas been recorded.

I claim:
 1. In a method for detecting an image signal representing aradiation image from a recording medium which has been exposed toradiation over a limited irradiation field in order to record aradiation image thereon, and detecting a prospective contour point,which is considered to be present on a contour of the irradiation field,from the image signal,a method for judging the correctness orincorrectness of a prospective contour point of an irradiation fieldwhich comprises the steps of:(i) judging whether a prospective contourpoint, which has been detected, satisfies or does not satisfy apredetermined judgement standard, wherein said judgement standard is therelationship between the distance from said detected prospective contourpoint to a predetermined point inside of the radiation image anddistances from other prospective contour points to said predeterminedpoint; and (ii) in cases where said detected prospective contour pointdoes not satisfy the predetermined judgement standard, judging that saiddetected prospective contour point is incorrect and is not present on aportion of an irradiated field.
 2. A method for judging correctness orincorrectness of a prospective contour point of an irradiation field asdefined in claim 1 wherein said recording medium is a stimulablephosphor sheet on which a radiation image has been stored, and saidimage signal is detected by exposing said stimulable phosphor sheet tostimulating rays which cause it to emit light in proportion to theamount of energy stored thereon during exposure to radiation, andphotoelectrically detecting the emitted light.
 3. A method for judgingthe correctness or incorrectness of a prospective contour point of anirradiation field as defined in claim 1 wherein said image signal isobtained by a preliminary read out operation.
 4. In a method fordetecting an image signal representing a radiation image from arecording medium which has been exposed to radiation over a limitedirradiation field in order to record a radiation image thereon, anddetecting a prospective contour point, which is considered to be presenton a contour of the irradiation field, from the image signal,a methodfor judging the correctness or incorrectness of a prospective contourpoint of an irradiation field which comprises the steps of:(i) judgingwhether a prospective contour point, which has been detected, satisfiesor does not satisfy a predetermined judgement standard, wherein saidjudgement standard is the relationship between distances from saiddetected prospective contour point to other prospective contour pointsand a predetermined threshold value; and (ii) in cases where saiddetected prospective contour point does not satisfy the predeterminedjudgement standard, judging that said detected prospective contour pointis incorrect and is not present on a portion of an irradiated field. 5.In a method for detecting an image signal representing a radiation imagefrom a recording medium which has been exposed to radiation over alimited irradiation field in order to record a radiation image thereon,and detecting a prospective contour point, which is considered to bepresent on a contour of the irradiation field, from the image signal,amethod for judging the correctness or incorrectness of a prospectivecontour point of an irradiation field, which comprises the steps of:(i)judging whether a prospective contour point, which has been detected,satisfies or does not satisfy a predetermined judgement standard,wherein said judgement standard is the relationship between an angle,which is formed between two straight lines, each of which connects saiddetected prospective contour point to one of two other prospectivecontour points, and a predetermined threshold value; and (ii) in caseswhere said detected prospective contour point does not satisfy thepredetermined judgement standard, judging that said detected prospectivecontour point is incorrect and is not present on a portion of anirradiation field.
 6. A method for judging the correctness orincorrectness of a prospective contour point of an irradiation field asdefined in claim 5 wherein said recording medium is a stimulablephosphor sheet on which a radiation image has been stored, and saidimage signal is detected by exposing said stimulable phosphor sheet tostimulating rays which cause it to emit light in proportion to theamount of energy stored thereon during exposure to radiation, andphotoelectrically detecting the emitted light.
 7. A method for judgingthe correctness or incorrectness of a prospective contour point of anirradiation field as defined in claim 5 wherein said image signal isobtained by a preliminary read out operation.
 8. In a method fordetecting an image signal representing a radiation image from arecording medium which has been exposed to radiation over a limitedirradiation field in order to record the radiation image thereon, anddetecting a prospective contour point, which is considered to be presenton a contour of the irradiation field, from the image signal,a methodfor judging the correctness or incorrectness of a prospective contourpoint of an irradiation field, which comprises the steps of:(i)investigating whether a prospective contour point, which has beendetected, satisfies or does not satisfy a predetermined judgmentstandard, (ii) calculating the mean image density in the regionextending from said detected prospective contour point to a point, whichis assumed to be present on a contour of an irradiation field from thepositions of other prospective contour points, and (iii) in cases wheresaid detected prospective contour point does not satisfy thepredetermined judgment standard and, at the same time, the mean imagedensity is lower than a predetermined threshold value, judging that saiddetected prospective contour point is incorrect and is not present onthe contour of the irradiation field.
 9. A method for judging thecorrectness or incorrectness of a prospective contour point of anirradiation field as defined in claim 8 wherein said judgment standardis the relationship between the distances from said detected prospectivecontour point to other prospective contour points and a predeterminedthreshold value.
 10. A method for judging the correctness orincorrectness of a prospective contour point of an irradiation field asdefined in claim 8 wherein said recording medium is a stimulablephosphor sheet on which a radiation image has been stored, and saidimage signal is detected by exposing said stimulable phosphor sheet tostimulating rays which cause it to emit light in proportion to theamount of energy stored thereon during exposure to radiation, andphotoelectrically detecting the emitted light.
 11. A method for judgingthe correctness or incorrectness of a prospective contour point of anirradiation field as defined in claim 8 wherein said image signal isobtained by a preliminary read out operation.
 12. In a method fordetecting an image signal representing a radiation image from arecording medium which has been exposed to radiation over a limitedirradiation field in order to record a radiation image thereon, anddetecting a prospective contour point, which is considered to be presenton a contour of the irradiation field, from the image signal,a methodfor judging the correctness or incorrectness of a prospective contourpoint of an irradiation field, which comprises the steps of:(i) judgingwhether a prospective contour point, which has been detected, satisfiesor does not satisfy a predetermined judgement standard, wherein saidjudgement standard is the relationship between the distance from saiddetected prospective contour point to a predetermined point inside ofthe radiation image and distances from different prospective contourpoints to said predetermined point; (ii) calculating the mean imagedensity in the region extending from said detected prospective contourpoint to a point which is assumed to be present on a contour of anirradiated field from the positions of other prospective contour points,and (iii) in cases where said detected prospective contour point doesnot satisfy the predetermined judgement standard and, at the same time,the mean image density is lower than a predetermined threshold value,judging that said detected prospective contour point is incorrect and isnot present on the contour of the irradiation field.
 13. A method forjudging correctness or incorrectness of a prospective contour point ofan irradiation field as defined in claim 12 wherein said recordingmedium is a stimulable phosphor sheet on which a radiation image hasbeen stored, and said image signal is detected by exposing saidstimulable phosphor sheet to stimulating rays which cause it to emitlight in proportion to the amount of energy stored thereon duringexposure to radiation, and photoelectrically detecting the emittedlight.
 14. A method for judging the correctness or incorrectness of aprospective contour point of an irradiation field as defined in claim 12wherein said image signal is obtained by a preliminary read outoperation.
 15. In a method for detecting an image signal representing aradiation image from a recording medium which has been exposed toradiation over a limited irradiation field in order to record theradiation image thereon, and detecting a prospective contour point,which is considered to be present on a contour of the irradiation field,from the image signal,a method for judging the correctness orincorrectness of a prospective contour point of an irradiation field,which comprises the steps of:(i) judging whether a prospective contourpoint, which has been detected, satisfies or does not satisfy apredetermined judgment standard, wherein said judgement standard is therelationship between an angle, which is formed between two straightlines, each of which lines connects said detected prospective contourpoint to one of two other prospective contour points, and apredetermined threshold value; (ii) calculating the mean image densityin the region extending from said detected prospective contour point toa point, which is assumed to be present on a contour of an irradiationfield from the positions of other prospective contour points, and (iii)in cases where said detected prospective contour point does not satisfythe predetermined judgement standard and, at the same time, the meanimage density is lower than a predetermined threshold value, judgingthat said detected prospective contour point is incorrect and is notpresent on the contour of the irradiation field.
 16. In a method fordetecting prospective contour points which comprises the steps of, incases where, in order to record a radiation image thereon, a rectangularrecording medium has been exposed to radiation over a limitedirradiation field having a rectangular shape so that two portions of thecontour of the irradiation field are approximately parallel to eachother and spaced at approximately equal distances from two oppositesides of the recording medium:(i) detecting an image signal representingsaid radiation image from said recording medium, (ii) sampling anddigitizing the image signal to obtain digital image signal componentsrepresenting image information stored at respective positions on saidrecording medium, (iii) carrying out differentiation processing on thedigital image signal components representing large information stored atpositions of the recording medium located along a single line whichintersects said two opposite sides of said recording medium at rightangles, and (iv) based on differentiated values obtained from thedifferentiation processing, detecting two prospective contour pointseach of which is considered to be present at each of two said contourportions, a method for judging the correctness or incorrectness of aprospective contour point of an irradiation field, which comprises thesteps of:(a) investigating whether the relationships expressed as

    |l.sub.1 -l.sub.2 |<α, l.sub.3 >β

are or are not satisfied, where l1 denotes the distance from one of saidtwo opposite sides of said recording medium to the detected prospectivecontour point, which is nearer to said one of said two opposite sides ofsaid recording medium than the other of two said detected prospectivecontour points, l2 denotes the distance from the other of said twoopposite sides of said recording medium to the detected prospectivecontour point, which is nearer to said other of said two opposite sidesof said recording medium than the other of two said detected prospectivecontour points, l3 denotes the distance between two said detectedprospective contour points, and α and β each denote a predeterminedthreshold value, and (b) in cases where at least one of saidrelationships is not satisfied, judging that two said detectedprospective contour points are incorrect and are not present on thecontour of the irradiation field.
 17. A method for judging thecorrectness or incorrectness of a prospective contour point of anirradiation field as defined in claim 16 wherein said recording mediumis a stimulable phosphor sheet on which a radiation image has beenstored, and said image signal is detected by exposing said stimulablephosphor sheet to stimulating rays which cause it to emit light inproportion to the amount of energy stored thereon during exposure toradiation, and photoelectrically detecting the emitted light.
 18. Amethod for judging the correctness or incorrectness of a prospectivecontour point of an irradiation field as defined in claim 16 whereinsaid image signal is obtained by a preliminary read out operation.